1. Field of the Invention
This invention relates to heat-sensitive recording materials, and especially to heat-sensitive recording materials capable of producing color images with improved storage stability. This invention is also concerned with novel phenol compounds, and in particular with novel phenol compounds useful as color-developing agents or additives for recording materials (for example, heat-sensitive recording materials) or as monomers or additives for high molecular materials.
Heat-sensitive recording materials making use of the color-producing reaction between an electron-donating chromogenic compound and an electron-attracting compound (color-developing agent) have been well known conventionally (for example, Japanese Patent Publication Nos. 4160/1968 and 14039/1970). These heat-sensitive recording materials are relatively inexpensive and, moreover, have the merit that recording equipment therefor are compact and maintenance-free. They have hence found utility in a wide variety of fields such as facsimiles, recorders and printers. PG,3
As electron-attracting compounds, phenol compounds are widely used. Among these, 2,2-bis(4'-hydroxyphenyl)propane (also called "bisphenol A") is extensively used for its availability at low cost. Bisphenol A is, however, accompanied by the drawback that heat-recording materials using bisphenol A as an electron-attracting compound cannot produce color images with good storage stability. In addition, heat-sensitive recording materials with bisphenol A as an electron-attracting compound also involve the drawback that their color-producing sensitivity is low. Heat-sensitive recording materials using, as an electron-attracting compound, a phenol compound other than bisphenol A have also been proposed, for example, 4-hydroxybenzoate esters (Japanese Patent Laid-Open No. 144193/1981 and Japanese Patent Publication No. 30640/1989), aralkyloxyphenols (Japanese Patent Publication No. 31678/1990 and Japanese Patent Laid-Open No. 225789/1985) and hydroxybenzophenones (Japanese Patent Laid-Open No. 193388/1982).
The use of 4-hydroxybenzoate esters, for example, benzyl 4-hydroxybenzoate as an electron-attracting compound, however, leads to the drawback that the density of a produced color image drops with time. It is also accompanied by the drawback that white crystals of benzyl 4-hydroxybenzoate are formed on the produced color image thereby to present a powdered appearance (the so-called "whitening phenomenon"). The use of aralkyloxyphenols, for example, 4-benzyloxyphenol as an electron-attracting compound also results in the drawback that the density of a produced color image drops as time goes on. When hydroxybenzophenones, for example, 4-hydroxybenzophenone is used as an electron-attracting compound, the resulting color image has poor storage stability (for example, hydrothermoresistance and waterproofness) so that they are not considered to have sufficient quality or properties for practical use.
On other hand, heat-sensitive recording materials containing--as a method for improving the color-producing sensitivity--a thermofusible compound (sensitizer) in addition to an electron-donating chromogenic compound and an electron-attracting compound are also widely used. Proposed as thermofusible compounds include terphenyls (Japanese Patent Publication No. 7958/1988), benzyl 4-benzyloxybenzoate (Japanese Patent Publication No. 30878/1988), naphthol derivatives (Japanese patent Publication No. 42590/1988), aminophenol derivatives (Japanese Patent Laid-Open No. 211494/1983), benzylbiphenyls (Japanese Patent Publication No. 11437/1990), diaryloxyalkane derivatives (Japanese Patent Laid-Open Nos. 56588/1985 and 16888/1986), oxalate ester derivatives (Japanese Patent Laid-Open No. 1583/1989), etc. However, it is the current situations that, although heat-sensitive recording materials containing one or more of these thermofusible compounds have been improved to some extent in color-producing sensitivity, they are accompanied by the problem of the extremely poor storage stability of produced color images, said storage stability being usually still inferior to that of color image produced without the addition of any thermofusible compound.
Accordingly, there is now a strong demand for the provision of heat-sensitive recording materials free of the drawbacks or problems described above, namely, for the provision of heat-sensitive recording materials capable of producing color images with excellent storage stability and, further, heat-sensitive recording materials having good color-producing sensitivity and capable of producing color images with excellent storage stability.